8-5. Reliability and Single Point Failure Design Considerations in Thermionic Space Nuclear Power Systems ELLIOT B. KENNEL, MARK S. PERRY & BRIAN D. DONOVAN Introduction Nuclear reactor-powered spacecraft must be designed according to accepted reliability measurement and forecasting techniques that are used by other spacecraft components. To be competitive with solar power systems, a high reliability will have to be achieved—in the range of 0.90-0.95 for a 7-year mission. In order to accomplish this high reliability, the nuclear reactor subsystem will have to have a ‘wearout' life substantially longer than seven years. For the first seven years of operating life, failures may be assumed to occur approximately at random, leading to where R(f) is the probability that the system remains operational as a function of time, and A is the reliability constant. For 2?(t) = 0.90 at 7 years, Z = O.O15O5. The mean time to failure (MTTF) is defined as the value of t at which R(f)=0.50 or Note that MTTF can be much longer than wearout life. For example, in the tire industry, random failure (blowout) may be much less frequent than wearout. However, equation (1) would not be used in the region close to wearout. System reliability is in turn related to the reliability of n crucial subsystems required for operability of the system by the expression Among the crucial subsystems for space reactor systems are the nuclear heat source, energy conversion, power conditioning, thermal transport, reactor control and heat rejection subsystems. These subsystems must have reliabilities in the range of 0.97 to 0.99 for 7 years, corresponding to MTTF of 300-400 years. Note also that confidence is very important for cases in which high reliability is desired and data are sparse. Confidence takes into account that the experimental distribution of data may differ from the real distribution. Consider a hypothetical case in which a single component is tested for seven years without a failure. Based on a Elliot B. Kennel, Mark S. Perry & Brian D. Donovan, WRDC/POOC-2 WPAFB, OH 45433-6523, USA. Paper number IAF-ICOSP89-8-5.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==